партнер по поединкам
Context: The speaker was jokingly mentioning someone he often has spars with.
The why of using the preposition "по" does not come naturally to me: "a partner for quarrels"? Could it be that "партнер" often, if not always, takes "по" as a preposition?
As an aside: Does "партнер по поединку" in the dative singular work just as well?
One of the meanings the "по" conveys is "pertaining to". "Заседание по поводу [чего-то]" -- "meeting to discuss/celebrate [something]"; "работы по данной теме" -- "[scientific] works on this subject"; "сосед по [лестничной] площадке" -- "neighbour whose flat's located at the same [stairs] landing".
The aside: your case with dative singular wouldn't work, I think, you'd want "партнер/противник в поединке/игре" here.
Yes, партнёр is often followed by по:
Singular (партнёр по поединку) would mean a partner just for the current round / fight, whereas партнёр по поединкам (plural) means a regular sparring partner.
